The categories in your blog are great for your users and make it easier for them to zero in on what they’re looking for. Did you know they were great for the Search Engines, as well?

Grouping posts about the same general topic together helps to theme your blog for the Search Engines. All the posts in a category appear together on the page(s) for that topic, along with good links and keywords. In addition, the posts generally link to other related posts, which gives added relevance.

However, duplicate content problems can arise if you do not use the built in WordPress optional excerpts feature and again if you place posts into more than one category. While it’s tempting to select all the possible categories for each post, choose just the one best category for each post, instead. If you must, go ahead and give it a second category, but this should be done only as an occasional exception and not as a standard practice. It is much better to have more categories than to place your posts into multiple categories.

Read more about how blog categories can cause problems.